1. Field of the Invention
The present invention relates to a continuous process for the environmentally friendly drying and purification of benzanthrone.
2. Description of the Prior Art
Benzanthrone, a dye intermediate, is produced in two different grades: crude and pure.
At present, as-synthesized benzanthrone is separated off on filter presses, washed with water and expressed to a solids content of about 50% by weight. The water-moist filter cake is then dried batchwise in vacuum paddle dryers at about 150.degree. C. and 600 mbar (absolute) to a residual water content of about 1% by weight. Dried benzanthrone crude still contains about 15% by weight of solid impurities, for example iron oxides and silicates, from the synthesis stage.
The subsequent purification of the dried, crude benzanthrone to give pure benzanthrone is carried out in sublimation furnaces at about 400.degree. C. and 1 mbar (absolute). To this end, the dried, crude benzanthrone is manually introduced into metal pans which are then put into the sublimation furnaces. On heating and evacuation the benzanthrone sublimes out of the metal pans and desublimes onto the water-cooled walls of the sublimation furnace, from where it has to be knocked off manually after every batch. The pans likewise need to be cleaned by hand after every batch to remove the hard impurities left behind.
The disadvantage of the process as practiced is that it involves handling strongly dusting, allergenic products, which lead to appreciable problems not only within the plant where the process is practiced but also in further processing plants. At the original plant the problem areas are the open, labor-intensive manipulation of the product when filling and emptying the vacuum paddle dryer, when introducing the dry material onto the metal pans, when cleaning these pans and when knocking the desublimed product off the walls of the sublimation furnace. This manual work represents an appreciable dust nuisance and contamination hazard to work personnel.